Field of the Invention
The invention relates to methods for recording a digitized audio signal and a telephone answering machine in which such methods can advantageously be employed.
In digital answering machines currently available on the market, the digitized signal of a recorded acoustical message is reduced from the data quantity that recorded with optimal conversion to a lesser data quantity in order to save on the cost of required memory. For storing digital data in memory, the analog speech signal is digitized at a predetermined sampling rate in a first step. The resultant data stream is compressed in a second step with the aid of a data or speech compression process. The data stream reduced by the data compression is then recorded in a third step in a digital memory. A system operating on this principle has been integrated for instance into the answering machine, type GIGASET 1030 available from the present applicant.
The ratio between the incoming data stream per unit of time (that is, the sampling rate of an A/D converter or of the digital speech information arriving in an ISDN telephone) and the data stream per unit of time present after the data compression is called the data compression rate. A high data compression rate accordingly achieves a major reduction in data volume as a result of the compression. However, with an increasing data compression rate the speech intelligibility drops. A compromise must be made between the magnitude of the data compression rate and the resultant speech intelligibility.
In currently available answering machines, the digital speech recording is done with a fixed data compression rate. Some equipment still allows manual variation of the data compression rate, but that must be specified by the user and cannot be controlled automatically. Other equipment allows retroactive data condensation of already recorded messages, in that the recorded messages are read out of the memory and subjected to data compression and then written back into the memory. This process is often called recompression.
A method for controlling signal recording for a digital answering machine is known from Published, Non-Prosecuted German Patent Application DE-OS 44 26 534. In the method, once again, the possibility of data compression is employed.
According to the method disclosed therein, the signal recording is controlled by regulating the ratio between speech quality and recording capacity as a function of the data quantity to be recorded. To that end, the data compression rate is set as a function of the still-available memory capacity, or data recompression is performed. However, then it is always an entire message that is subjected to data compression.
From Japanese Patent Disclosure JP-8-98134, a data recording device has become known in which the digitized video or audio data are recorded in a recording medium at a variable compression rate. The compression rate is made dependent on the data information quantity with the aid of a discriminator circuit. This entails a great expense for circuitry.